This invention relates generally to the field of radio frequency (RF) communications, and, more specifically, to an improved technique for quickly adjusting the frequency offset bias of a non-coherent 0.2 Gaussian minimum shift keying (GMSK) receiver detector. The invention is particularly useful for radios operating in a time division multiplexed (TDM) communication system.
Recently, there has been an increased demand for voice, digital voice, and high-speed data communications over land mobile radio channels. Since the radio frequency spectrum is inherently limited, one must devise new system concepts and organizational features to accommodate the increased demand for mobile and portable radio communications services. Time-division multiple access (TDMA) is one method of achieving more efficient spectrum utilization.
In its simplest form, a TDMA system is comprised of a transmitting base station capable of time-multiplexing messages from at least two users on a single RF channel, and one or more remote receiving stations capable of receiving at least one of the time-multiplexed messages. Typically, the receiving station would be a mobile or portable unit capable of transmitting a TDM message to the base station (or repeater station) on a second (or the same) RF channel. Refer to U.S. Pat. No. 4,742,514 entitled "Method and Apparatus for Controlling a TDM Communication Device", for further information regarding TDMA.
In a TDM system, the base station (or repeater station) must be capable of receiving short bursts of information from all mobile and portable units (remote units) within the system. Each of these remote units may transmit on slightly different frequencies due to different environmental factors, e.g., power supply voltage fluctuations, ambient temperature variations, etc. Each mobile or portable unit may also transmit on slightly different frequencies due to differences in their internal frequency references. In a frequency modulation (FM) receiver, this frequency difference translates into a DC offset bias voltage at the detector. In order for the base station to receive all of the TDM information burst from a remote unit, the base station receiver must quickly adjust its receive frequency offset to the transmitted frequency of the incoming signal.
This requirement for fast frequency acquisition often conflicts with the requirement to receive low frequency information present in the TDM burst. This conflict can readily be observed in a TDM system using FM to transmit the data. If the TDM data contains low frequency information, e.g., when the information is non-return-to-zero (NRZ) data, or if it contains low frequency signalling tones, e.g., sub-audible digital private line (DPL.TM.), or if it uses an analog method to scramble speech, e.g., frequency-inversion voice scrambling, the FM receiver discriminator must be coupled at a very low frequency to the information processing circuits, in order to pass the lowest frequency of interest. A large valued AC coupling capacitor is often used to series couple digital information from the discriminator to a limiter, such that a microprocessor can then be used to decode the data. Such low frequency AC coupling requires a significant amount of time for the receiver circuitry to adjust to any frequency offset between the transmitter and receiver. Depending on the actual frequency offset, this acquisition time can be more than the entire length of the TDM burst.
Various methods have been used to adjust the receiver frequency to the transmitter frequency in an FM radio communication system. U.S. Pat. No. 4,518,922 utilizes one common approach, which is the use of an automatic frequency control (AFC) circuit, in which the incoming signal is averaged over a period of time in order to find the frequency error between the transmitter and receiver. An error signal is then used to adjust the receiver local oscillator to the transmitted frequency. However, AFC circuits are generally not suitable for TDM systems, due to the long time averages which must be taken to find the frequency offsets. Again, this averaging (acquisition) time may be longer than the entire length of the TDM burst.
Using another approach, U.S. Pat. No. 4,575,863 discloses a programmable bias circuit for quickly adjusting the data limiter offset bias to a received FSK data stream. This method utilizes a fast time constant to determine the center frequency of the received signal for the limiter when the data receiver is idle, and then switches to a slow time constant after word synchronization has been detected. However, any such "center-slicer" based approach will not work in those TDM systems which don't transmit digital data in the information time slot (e.g., wherein analog voice information is sent instead of digital vocoded speech information). Moreover, it is not practical to use this approach in TDM systems transmitting data using modulation techniques which do not lend themselves to detection using a center frequency reference (e.g., 0.2 GMSK, having a small "eye opening").
A need, therefore, exists to provide a method and means for fast frequency acquisition of a TDM transmitted signal by the receiver, while at the same time allowing low frequency coupling of the received information to the information processing circuits.